90s formula change

[Cowmatian]

After working on my '93 aristocrat (basically smelled like kerosene), I thought I would be able to tell if a lamp had a good seal or not by smell. I ran my yellow aristocrat from 2000, and smelled nothing. Then I noticed a hissing sound, and saw that liquid  was bubbling out from under the bottle cap. It has no smell (maybe slightly soapy), but not even a hint of the kerosene smell. I assume that the 2000 bottle was made with safer ingredients, but I'm not sure. Anyway does anyone know about formula variations throughout the '90s to early 2000s? Has anyone noticed certain smells while working on '90s to early 2000s lamps? I remember hearing somewhere that ethylene glycol was removed from the fluid in the early '90s. Since the flow of my lamps from the 90's to early 2000s seems to be consistent throughout the years, I would like to go with the safer lamps.